This invention relates to an electromagnetic wave absorbing thermoconductive silicone gel molded sheet. More specifically, this invention relates to an electromagnetic wave absorbing thermoconductive silicone gel molded sheet used for absorbing electromagnetic wave noise of an exoergic electronic part and the like, and for radiating heat.
Recently-developed electronic apparatuses such as computers, personal computers and television sets generate various electromagnetic wave noises of high frequencies, and cause problems of electromagnetic interference (EMI). To solve such problems, techniques for absorbing electromagnetic waves from electronic parts of the electronic apparatuses have been searched.
Various techniques have been disclosed, including Unexamined Published Japanese Patent Application (Tokkai-Sho) No. 50-155999, wherein electromagnetic waves are absorbed by a composite comprising metal magnetic particles and resin. Such an electromagnetic wave absorber, however, does not have sufficient absorbency, and thus, sufficient effects cannot be obtained when the absorber is attached to an electronic part that generates electromagnetic waves.
Another disclosure is an electromagnetic wave absorbing sheet comprising a resin layer filled with metal oxide magnetic particles formed on an electroconductive material (Tokkai-Sho 57-129003, Tokkai-Hei 4-234103, and Tokkai-Hei 7-249888). The sheet decays electromagnetic waves by reflecting some of the electromagnetic wave and absorbing other parts thereof. The complicated structure of the sheet, however, raises the production cost. Moreover, the applicable temperature range is narrow because the heat resistance of the resin is low.
Thermoconductive sheets have been disclosed as a part of cooling systems for heat-radiation in exoergic electronic parts of electronic apparatuses (Tokkai-Hei 2-166755, Tokkai-Hei 2-196453, and Tokkai-Hei 6-155517). Many exoergic electronic parts, however, simultaneously have electromagnetic interference caused by electromagnetic wave noise. In such a case, a shielding material, other than the thermoconductive sheet, is used to absorb electromagnetic waves. Since plural kinds of parts are necessary, the cost will increase.
In order to solve the above-mentioned problems, this invention provides an electromagnetic wave absorbing thermoconductive silicone gel molded sheet that has a high electromagnetic wave absorbing property and thermoconductivity at a low cost.
An electromagnetic wave absorbing thermoconductive silicone gel molded sheet to achieve the purpose is formed from silicone gel comprising metal oxide magnetic particles and a thermoconductive filler.
The sheet of this invention has a high electromagnetic wave absorbing property and thermoconductivity compared to a combined use of a conventional electromagnetic wave absorbing sheet and a radiation sheet, since the sheet of this invention comprises metal oxide magnetic particles and a thermoconductive filler. The reason can be explained as follows. When electromagnetic energy is absorbed in an electromagnetic wave absorbing sheet, it is converted into heat energy usually. As the sheet of this invention has thermoconductivity, the heat energy is transferred rapidly in the sheet and radiated. Therefore, the sheet of this invention rapidly converts electromagnetic energy into heat energy and thus, demonstrates a high electromagnetic wave absorbing property. As a result, the sheet of this invention can be used alone because of its electromagnetic wave absorbing property and thermoconductivity. The simple structure and low cost also contribute to its usefulness. In addition, the sheet of this invention can be used in a wide temperature range since the silicone gel resists to higher temperature compared to other organic plastics, and it is excellent in thermostability.
It is preferable in the sheet that a sheet of an electroconductive reinforcement is embedded in at least either an upper or lower surface of the silicone gel molded sheet, since a higher electromagnetic wave absorbing property is obtained. An electromagnetic wave entering the sheet is reflected by the electroconductive sheet and passed through the sheet again to provide the above-mentioned actions and effects. In other words, electromagnetic energy will be converted into heat energy when it is absorbed in the electromagnetic wave absorbing sheet. In the sheet of this invention, provided with thermoconductivity, the heat energy is transferred rapidly in the sheet and radiated. As a result, electromagnetic energy is soon converted into heat energy in the sheet of this invention, and demonstrates a high electromagnetic wave absorbing property. For this reason, the electromagnetic energy is converted into heat energy again with efficiency.
Although the electromagnetic wave absorbing thermoconductive silicone gel molded sheet has low hardness and less strength, the embedded electroconductive reinforcement reinforces the sheet, and thus, the operability in amounting step is improved.
A method for producing an electromagnetic wave absorbing thermoconductive silicone gel molded sheet of this invention comprises the steps of:
adding metal oxide magnetic particles and a thermoconductive filler to a silicone gel and mixing them to prepare a mixture of a uniform composition;
degassing the mixture with a deaerator;
vertically sandwiching the degassed mixture with supporting films of a predetermined shape; and
press-molding the mixture while heating to harden the mixture to be a sheet. A silicone gel molded sheet of this invention can be produced rationally and efficiently in this method.
In the above-mentioned method, the step of sandwiching the degassed silicone gel mixture vertically with the supporting films of a predetermined shape is preferably carried out by:
placing an unhardened silicone gel blended with metal oxide magnetic particles and a thermoconductive filler on a lower supporting film of a predetermined shape;
covering the silicone gel with a sheet of electroconductive reinforcement and an upper supporting film; and
embedding the sheet of electroconductive reinforcement in the silicone gel.
As a result, the silicone gel molded sheet in which the electroconductive reinforcement is embedded can be produced efficiently and rationally.